1. Field of the Invention
The present invention relates to a semiconductor device having a circuit including thin film transistors (hereinafter referred to as TFTs) and a manufacturing method of the semiconductor device. For example, the present invention relates to an electro-optical device typified by a liquid crystal display panel, a light-emitting display device having organic light-emitting elements, a plasma display device, and an electronic device having an optical module or a wireless chip as a component.
Note that “semiconductor device” used in this specification refers to devices in general that can operate by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and an electronic device are all included in the semiconductor device.
2. Description of the Related Art
In recent years, much attention has been given to a technique for forming thin film transistors (TFTs) using a semiconductor thin film (with a thickness of approximately several to several hundred nanometers) which is formed over a substrate having an insulating surface. Thin film transistors have been widely applied to electronic devices such as ICs and electro-optical devices, and their development especially as switching elements for an image display device has been accelerated.
As image display devices, liquid crystal display devices have been known. Active-matrix liquid crystal display devices have been commonly used because they can provide images with higher definition than passive-matrix liquid crystal display devices. In an active-matrix liquid crystal display device, display patterns are formed on a screen by driving of pixel electrodes that are arranged in a matrix. Specifically, voltage is applied between a selected pixel electrode and a counter electrode corresponding to the pixel electrode, and thus, a liquid crystal layer located between the pixel electrode and the counter electrode is optically modulated. This optical modulation is recognized as a display pattern by a viewer.
The present applicant has proposed liquid crystal dropping in Reference 1 (U.S. Pat. No. 4,691,995).
In recent years, light-emitting devices having EL elements as self light-emitting elements have been researched actively, and in particular, light-emitting devices using an organic material as an EL material have attracted attention. These light-emitting devices are also called EL displays.
A phenomenon in which light emission occurs when an electric field is applied to a substance is referred to as an electroluminescence (EL) phenomenon, which is a known phenomenon. In particular, inorganic EL using an inorganic thin film of ZnS: Mn and organic EL using an organic thin film formed by evaporation are bright, exhibit EL emission with high efficiency, and are attempted to be applied to displays.
The luminescence of an organic compound includes light emission (fluorescence) upon returning to a ground state from a singlet-excited state and light emission (phosphorescence) upon returning to a ground state from a triplet-excited state. Light-emitting devices manufactured using a film formation apparatus and a film formation method of the present invention can be applied to cases where they perform either light emission.
Light-emitting devices are of a self light-emitting type, unlike liquid crystal display devices; therefore, a feature of the light-emitting devices is that they have no problem with viewing angle. That is, light-emitting devices are more suitable displays for outdoor use than liquid crystal displays, and a variety of applications have been proposed.
In addition, light-emitting elements each including a cathode, an EL layer, and an anode are called EL elements, which include two types: one is that in which an EL layer is formed between two kinds of stripe electrodes that are arranged at right angles to each other (simple-matrix type), and the other is that in which TFTs are provided and an EL layer is formed between pixel electrodes and counter electrodes that are arranged in a matrix (active-matrix type). However, when pixel density is increased, it is thought that the active-matrix type where each pixel (or each dot) is provided with a switch be advantageous because the active-matrix type can be driven at lower voltage.
An EL material forming an EL layer deteriorates very easily and deteriorates by easily oxidizing or absorbing moisture under the presence of oxygen or water. Therefore, there are problems in that the luminance of light emission and the lifetime of a light-emitting element decrease. Thus, by covering of a light-emitting element with a sealing can, enclosure of dry air inside the sealing can, and attachment of a drying agent, oxygen or moisture is prevented from reaching the light-emitting element.
The present applicant has also proposed, in Reference 2 (U.S. Pat. No. 6,724,150), that a light-emitting device is provided with a drying agent in a depression in a sealing substrate.
In addition, the present applicant has proposed, in Reference 3 (U.S. Pat. No. 6,876,145), that a semiconductor substrate is used for a light-emitting device.